1. Field of the Invention
The present invention relates to a Phase-change Random Access Memory (PRAM) device and a method of manufacturing the same, and more particularly, to a PRAM device having high reliability and excellent operating characteristics by including a heating layer having optimal heating characteristics and a method of manufacturing the same. This work was supported by the IT R&D program of MIC/IITA. [2005-S-072-02, Technology of a nano scale phase change data storage]
2. Description of the Related Art
Phase-change Random Access Memory (PRAM) devices are memory devices using a characteristic that electrical resistance of a phase change material formed of a chalcogenide compound including the elements Se, Te, and S is changed according to its crystalline phase. That is, the crystalline phase of the phase change material can be reversibly switched between a crystalline state and an amorphous state according to a voltage or current applied to the phase change material, each state having a different electrical resistance. Such PRAM devices are non-volatile, fast, and can be highly integrated.
Since electrical resistance of a phase change material is significantly changed according to whether the phase change material is in a crystalline or an amorphous state, physical properties of the phase change material can be used to apply the phase change material for use as information storing devices. More specifically, a PRAM memory device is operated according to a set process and a reset process described below. The set process comprises changing a phase change material layer of the PRAM memory device from an amorphous state with high resistance to a crystalline state with low resistance. The reset process comprises changing the phase change material layer of the PRAM memory device from a crystalline state to an amorphous state. For the set and reset processes, two types of current pulses having different amplitudes and times are needed. A reset pulse having a high amplitude and short time increases the internal temperature of a phase change material layer above a melting point so that a crystalline structure of the phase change material layer is maintained in a liquid state, and is cooled down after the pulse is completed, so that the crystalline structure of the phase change material layer is changed into an amorphous state. A set pulse having a relatively lower amplitude and longer time than the reset pulse increases temperature of the phase change material layer above a crystallization temperature so that a crystalline structure of the phase change material layer is changed into a crystalline state.
FIGS. 1A and 1B are side cross-sectional views of a conventional PRAM device. Referring to FIG. 1A, the conventional PRAM device includes a lower electrode 10 and an upper electrode 50 respectively disposed on the lower part and the upper part of a phase change material layer 40 and a heating layer 20 disposed to contact the phase change material layer 40. Also, portions of each of the phase change material layer 40 and the heating layer 20 required to be insulated from each other are insulated by an interlayer insulation film 30. Such structure may selectively have a structure where a heating layer 22 having a small cross section is disposed below a phase change material layer 42, as illustrated in FIG. 1B. Here, a lower electrode 12, an upper electrode 52, and an interlayer insulation film 32 may correspond to the lower electrode 10, the upper electrode 50, and the interlayer insulation film 30 illustrated in FIG. 1A, respectively.
An operation of the conventional PRAM device is described below with reference to FIGS. 1A and 1B.
When a current is applied through the lower electrodes 10 and 12 and the upper electrodes 50 and 52, joule heat is generated from the heating layers 20 and 22 and phases of some regions of the phase change material layers 40 and 42 are changed. Since heat generated from the phase change material layers 40 and 42 may not be sufficient to cause phase change, the heating layers 20 and 22 are employed in this regard. Accordingly, in general, the heating layers 20 and 22 may be formed of materials having high electrical resistance which can generate a sufficient amount of heat.
TiN, TiAlN, and TiSiC are widely used as the materials for the heating layers 20 and 22, and W can also be used as a material for the heating layers 20 and 22. For these materials to be used as the heating layers 20 and 22, they should have high electrical resistivity (ρ), low thermal conductivity, and low specific heat. The electrical resistivity should be high to generate a large amount of heat, thermal conductivity should be low to reduce unnecessary heat loss to the outside, and specific heat should be low to cause large temperature changes. The most important parameters of these properties are electrical resistivity and thermal conductivity.
Conventionally, such heating layers 20 and 22 are formed of a single material. However, materials that satisfy the above conditions have not yet been found. Accordingly, heating layers using excellent physical properties of conventionally known materials are required.